Lithium ion secondary batteries having high voltage and high energy density have been widely used as power sources for electronic devices such as notebook computers, cellular phones, and Audio-Visual devices. Lithium ion secondary batteries belong to non-aqueous electrolyte secondary batteries using non-aqueous solution-based electrolyte as electrolyte, and carbon materials capable of absorbing and desorbing lithium are generally used as negative electrode active material. As positive electrode active material, lithium-containing composite oxide (LiCoO2 etc.) is used.
In recent years, as smaller electronic devices having higher performance have been realized, demand for lithium ion secondary batteries having higher capacity and longer life has been increasing. Also, as use frequency of electronic devices has been increasing along with development of ubiquitous society, demand for shortening of charge time has been increasing greatly.
In order to achieve secondary batteries having higher capacity, it is generally effective to increase the filling density of active material. In lithium ion secondary batteries, however, if the filling density of the active material is increased, lithium ion-acceptability of active material is likely to decline during the charge. As a result, the charge and discharge cycle life characteristics (cycle characteristics, hereinafter) may be deteriorated.
Meanwhile, in order to achieve non-aqueous electrolyte secondary batteries having longer life, that is, improved cycle characteristics, it has so far been proposed to reduce the charge current. By reducing the charge current, it is possible to prevent deterioration in the cycle characteristics even when the active material is filled at high density.
Further, when the upper limit of the charge voltage is high, decomposition of the non-aqueous electrolyte is accelerated, which leads to deterioration in the cycle characteristics. Therefore, deterioration in the cycle characteristics can be prevented by suppressing the upper limit of the charge voltage.
However, for example, if the charge current is reduced, the quantity of electricity that can be charged to secondary batteries per unit time is decreased, and the charge time naturally becomes longer. Shortening of the charge time of secondary batteries has been demanded in various fields. Therefore, this demand cannot be met if the charge current is reduced simply. Meanwhile, if the upper limit of the charge voltage is suppressed, the discharge capacity is reduced, whereby the time during which devices can be used by one charge is shortened.
Therefore, in order to shorten the charge time without deteriorating the cycle characteristics of secondary batteries, various charge methods have so far been proposed. For example, Patent Literature 1 proposes firstly charging a lithium ion secondary battery at a high constant current, reducing the charge current when the terminal voltage of the battery reaches a predetermined cutoff voltage to lower the battery voltage, and also switching the cutoff voltage after switching of the current according to a fall of voltage due to internal resistance of the battery.
Also, Patent Literature 2 proposes repeating a procedure of charging firstly a lithium ion secondary battery at a high constant current until the battery voltage reaches a predetermined value, and reducing the current when the battery voltage reaches the predetermined voltage to lower the battery voltage.